Optically imaging systems are temperature-dependent. Due to the effect of the temperature, not only the geometry but also the refractive index of the material of optical components changes. Especially in the case of lenses, whose thickness, radius and refractive index are decisive for the imaging, temperature fluctuations can considerably influence the imaging quality. Moreover, an optical system is also affected by the thermal expansion of the mounting parts that hold the optical components together.
In this context, various approaches are known that compensate for the effect of the temperature on optical components and on the optical systems made up of such components.
One approach is to specifically make the individual optical components and the mounting parts out of materials that have different coefficients of expansion and to then combine these materials with each other in such a way that their coefficients of expansion act in opposite ways. In particular, there are modules of this type including several lenses. This type of temperature compensation is restricted to only small temperature ranges.
Another approach is that changes in the imaging properties of an optical system, which are especially due to a change in the focal length, are compensated for in that the optical components that are especially relevant for this are moved within the optical system. U.S. Pat. No. 4,162,120 shows a device with a fixed holder and a lens group that is axially moved inside the fixed holder as a function of temperature.
For this purpose, there is fundamentally an outer mounting part joined to a housing of the optical system and an inner mounting part that supports the optical component, whereby the two mounting parts are arranged coaxially to each other and have a shared axis along which the optical component is moved.
Various technical solutions are used to effectuate the movement, and these differ primarily in terms of whether the movement is done actively or passively.
For an active movement, it is possible to employ temperature sensors to control actuators such as, for example, a motor with a rack and pinion drive, with a spindle drive or with a piezoelectric drive. With this approach, it is possible to achieve large actuating forces and long travel distances, although this means that a large installation space will be required. Moreover, such devices require a power supply, something that is not feasible for many optical systems.
German patent application DE 10 2006 046 416 B4 describes a device that implements a passive movement by means of a cylinder-piston arrangement.
The outer mounting part is configured here as a cylinder firmly affixed to the housing, while the inner mounting part that supports an optical component is in the form of a piston. The cylinder and the piston are sealed off with respect to each other and together, over a prescribed length, they enclose a chamber filled with a fluid. As a function of the temperature coefficient of the fluid and the length of the chamber, the piston is moved in the axial direction inside the cylinder when the temperature of the fluid changes. Thus, in order to effectuate a certain movement of the optical component as a function of the temperature change, a certain length has to be prescribed for the chamber as a function of the temperature coefficient of the fluid used.
A drawback of this solution is that the conversion ratio between the expansion of the fluid and the adjustment distance is only 1:1, and only with a chamber of great length and thus a construction of great length is a possible adjustment range achieved of the kind that is needed for wider temperature ranges. A reactive polymer system that is liquid in the uncured state and that has a gel-like to elastomeric consistency in the cured state may be employed as the fluid. Such a device is not suitable for optical systems that work with extreme ultraviolet (EUV) radiation since the use of organic substances is excluded in order to avoid contaminations.
U.S. Pat. No. 5,557,474 describes a device for passive thermal compensation in which two lenses are moved with respect to each other as a function of the temperature change in order to jointly compensate for a temperature-dependent change in the imaging.
In order to compensate for a temperature-dependent change in the focal position of a system consisting of two individual lenses, the distance between the two lenses should be moved along the optical axis by 0.007 inches (0.17 mm) per change of 30° C. in the temperature.
For this purpose, spacer rings which are made of materials having different coefficients of expansion and which have planar surfaces that are wedge-shaped with respect to each other are arranged alternatingly between the two lenses. Depending on whether the spacer rings with the wedge-shaped surfaces tapered towards the outside or the spacer rings with the wedge-shaped surfaces tapered towards the inside have the larger coefficient of expansion, the distance between the lenses is decreased or increased when the temperature rises.
The magnitude of the change results from the number of spacer rings, from their wedge angles and radii as well as from the difference of the temperature coefficients of the materials.
Since the radial extension of the spacer rings is proportional to their radius and since the radius should be selected so as to match that of the lenses, such a device is only suitable for large optical systems, that is to say, lenses having a large diameter. A rough calculation shows that, assuming that the wedge surfaces have an angle of 2×45°, in other words, that both sides of the ring are configured to be wedge-shaped, and assuming that the difference between the coefficients of expansion is 23×10−6 K−1, then a ring having a diameter of 236 mm is needed for the envisaged compensation effect. Since a wedge angle of 45° can be expected to cause friction that will impair the function, a plurality of such rings will be required in the case of the usual friction values. Fundamentally speaking, this solution entails the disadvantage that a stick-slip effect is unavoidable owing to dry friction, and this markedly impairs the reproducibility and can completely prevent the function in the case of very small temperature fluctuations.
European patent application EP 0 604 328 A1 describes an optical array that yields a collimated light beam of the same quality over a wide temperature range in that a thermal compensation means is arranged between the mount of the collimating optical component and the holder of the light source. The thermal compensation means can be a compensation ring, formed by a laminate of layers in which bundles of fibers are wound at different angles. This arrangement is not suitable for use in the EUV range since it normally requires the use of an organic matrix as the composite material, and this has to be fundamentally ruled out for applications in the EUV range in order to avoid contaminations.
U.S. Pat. No. 4,162,120 likewise describes a thermal compensation device that effectuates a linear movement in response to temperature changes and that can be employed in optical systems.
In comparison to the state of the art described here, it should also be possible to compensate for large temperature changes in that a link assembly is constructed from a plurality of elongated links of two types having dissimilar coefficients of thermal expansion.
The first type of link is made of a material having a relatively high coefficient of thermal expansion such as aluminum or magnesium, while the second type of link is made of a material having a relatively low coefficient of thermal expansion such as Kovar or Invar.
The links are arranged alternately and joined to each other at their ends in order to form a folded link assembly. For the sake of clarity, one link with a low coefficient of expansion joined to one link with a high coefficient of expansion should be understood as a link pair.
For use in an optical system, for example, three such link assemblies—each consisting of a prescribed, equal number of link pairs—are mounted with one end on a fixed holder and with the other end on a movable lens mount.
The folding results in a summing effect of the length differences between the two links of each link pair caused by the temperature change.
This allows a length change equal to the difference resulting from the total length of all of the links of the first type minus the total length of all of the links of the second type.
A drawback of this approach is the amount of assembly work needed and the risk of a tilting of the lens due to tolerances between the individual link assemblies. Consequently, the lens or the optical element has to be held by a separate axial guide means. The achievable compensation distance is limited by the number of elements that can be placed along the circumference, that is to say, by the effective diameter of the arrangement. A further increase in the compensation distance per Kelvin (K) of temperature change can only be achieved with links having a greater length. This requires a considerable amount of space. The utilization of the available installation space is unfavorable. The use of link assemblies causes the arrangement to have a low level of stiffness in the lengthwise direction since the individual links bend under axial load. The give adds up with every link. A variable position of the optical system in space can be expected to be associated with a greater imprecision of the axial position of the lens. The lower intrinsic frequency in the lengthwise direction caused by the reduced stiffness is likewise disadvantageous.